This invention relates generally to laser ablation of fluoropolymer composite materials. More particularly, this invention relates to the laser ablation of fluoropolymer composites using ultraviolet radiation to form holes in circuit substrates for producing plated through-holes and plated vias.
As is well known, during circuit manufacturing, holes are needed to be formed through the circuit substrate and conductive layers. These holes are used to form vias which interconnect different conductive layers in a multi layer circuit laminate; and to produce plated through holes needed to interconnect spaced, opposed circuit layers in a diclad laminate.
Often, there is a need to form such holes with very small diameters (typically 25-200 .mu.m) through the generally copper clad laminates. In addition, to keep manufacturing costs low, these small diameter holes need to be formed at a reasonably fast rate (e.g. about one hole per second). Unfortunately, such holes are difficult or impossible to produce by mechanical drilling techniques.
Presently, infrared laser radiation (using a CO.sub.2 laser) is used to drill vias in fluoropolymer substrates. At the infrared wavelength of the CO.sub.2 laser (10.6 .mu.m), the fluoropolymer is absorptive and can be readily ablated. However, due to the long wavelength, the ablation is purely thermal. The hole walls are often roughened and damaged, which limits the possible resolution. In addition, at the CO.sub.2 laser wavelength, a thin carbonaceous residue is left on the copper or the hole walls and often on the surface surrounding the hole which then requires further cleaning by expensive processes such as plasma cleaning and the like. It will be appreciated that plasma cleaning can be expensive and problematic.
Ablation by ultraviolet radiation (as opposed to infrared radiation) has been proposed for etching polyimide circuit materials (see U.S. Pat. No. 4,508,749 to Brannon et al). However, UV radiation to form holes in fluoropolymeric circuit laminate materials has generally been thought unfeasible because fluoropolymers do not absorb UV (200-400 nm) radiation. For example, in an article entitled "Optical Absorption of Some Polymers in the Regions 240-170 nm" by Philipp et al (Appl. Phys, Lett. 48(2), 12 Jan. 1986), the authors provide evidence that absorption coefficients at 193 nm for polytetrafluoroethylene is about 2.times.10.sup.2 cm.sup.-1 as opposed to, for example, polyimide which is 4.times.10.sup.5 cm.sup.-1. Similar results using a laser at wavelengths of 193 nm and 248 nm were reported in an article entitled "Formation of Polymer Films by Pulsed Laser Evaporation" by Hansen et al, (Appl. Phys. Lett 52(1), 4 Jan. 1988). Thus, while it is quite apparent that polyimide materials such as those disclosed in U.S. Pat. No. 4,508,749 could be ablated by ultraviolet radiation, it is generally held that UV radiation cannot be practically used for ablation of fluoropolymer materials.